Many digitizers use the electromagnetic coupling between grid wires imbedded in a tablet and a cursor coil as a means for developing the electric signals used in determining the X and Y coordinates of the cursor position. In most cases the electrical signals induced in the grid windings are phase shifted and combined to produce a signal whose phase shift relative to the cursor coil excitation current is a linear function of cursor position.
Typically, the phase shift increases linearly from 0 to 360 degrees as the cursor coil moves through a distance equal to the pitch distance of the grid winding. When cursor movement continues in the same direction the phase changes abruptly to zero and increases linearly with position to 360 degrees as the cursor moves through successive intervals of length equal to the pitch distance. The position signal is therefore cyclical and can be used to accurately determine the position of the cursor coil within any given cycle. In order to provide absolute position information, the location of the particular cycle must be known as well as the position of the cursor coil within the cycle.
A convenient method of providing an absolute position signal for identifying individual cycles is to construct two position systems similar in every respect except for their pitch distances which differ fractionally by a small distance .DELTA.d. Subtracting the position measurement derived from one system from the measurement derived from the other produces a signal which cycles over longer intervals of movement than either of the two signals from which it is derived.
FIG. 1 shows two cyclical position signals produced by measuring phase shift. Signal 1 is obtained from a grid winding with a pitch distance of D and signal 2 is obtained from a grid winding with a winding pitch distance of D+.DELTA.d. The signal 3 obtained by subtracting 1 from 2 is discontinuous, going through an abrupt change every time either signal 1 or signal 2 completes a cycle. In order to obtain a simple continuous linear relationship between cursor coil position and phase shift, it is necessary to add the maximum amplitude of signal 1 to signal 3 whenever signal 2 is greater than signal 1. Signal 4 is the result of performing this addition on signal 3. Signal 4 can be used to provide an indication of cursor absolute position or more commonly it is combined with the information available from either signal 1 or 2 to form a course/fine system with superior absolute positioning accuracy.
A discussion of an absolute position coordinate determining device of the type involved in the present application is contained in U.S. Pat. No. 3,735,044. The technical description contained in that patent is incorporated into this application as background information to the same extent as if fully set forth herein.